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Abstract:

The invention relates to an electrode for an electrochemical cell which
exhibits good electron conductivity and good chemical conductivity, as
well as good cohesion with the solid electrolyte of the electrochemical
cell. To do this, this electrode is made from a ceramic, which is a
perovskite doped with a lanthanide having one or more degrees of
oxidation and with a complementary doping element taken from the
following group: niobium, tantalum, vanadium, phosphorus, arsenic,
antimony, bismuth.

Claims:

1. An Electrode for an electrochemical cell with mixed electron and
proton conductivity, said electrode comprising a ceramic, said ceramic
being a perovskite doped with a lanthanide with one or several degrees of
oxidation, characterised in that said ceramic is doped with an additional
doping element taken among the group composed of niobium, tantalum,
vanadium, phosphorus, arsenic, antimony, bismuth.

2. The Electrode according to claim 1, also comprising a metal, the metal
and the ceramic forming a cermet.

3. The Electrode according to claim 1, in which the perovskite used is a
zirconate.

4. An Electrochemical cell comprising two electrodes according to claim 1
and a solid electrolyte arranged between the two electrodes.

5. The Electrochemical cell according to claim 4, in which the solid
electrolyte is made from a perovskite doped with a lanthanide with one
degree of oxidation, the perovskite used in the solid electrolyte being
of the same nature as that used in the electrodes.

6. A method of making an electrode according to claim 1, comprising the
following steps: (a) Synthesis of a perovskite powder doped with a
lanthanide with one or several degrees of oxidation; (b) Synthesis of a
powder of an additional compound comprising a doping element taken among
the group composed of niobium, tantalum, vanadium, phosphorus, arsenic,
antimony and bismuth, the additional compound being such that the degree
of oxidation of the doping element in this additional compound is greater
than or equal to 5; (c) Mix the doped perovskite powder and the
additional compound; (d) Sinter this mix.

7. The Method according to claim 6, in which sintering is done in an
almost non-oxidising atmosphere. (Currently Amended) The Method according
to claim 6, in which the perovskite powder and the powder of the
additional compound are also mixed with a metallic powder or a metallic
phase precursor.

8. The Method according to claim 6, also comprising a step between steps
(c) and (d), in which a stack is made comprising at least two layers
formed from a mix of the doped perovskite powder and the additional
compound, between which there is an interlayer comprising a layer of
perovskite powder.

9. The Method according to claim 8, in which the stack also comprises two
intermediate layers, each intermediate layer being located between the
interlayer and one of the two layers formed from the mix of the doped
perovskite powder and the additional compound.

10. A Method of manufacturing an electrode according to claim 1, the
method comprising the following steps: (a) Direct synthesis of a
perovskite powder doped with a lanthanide with one or several degrees of
oxidation containing an additional compound comprising a doping element
taken among the group composed of niobium, tantalum, vanadium,
phosphorus, arsenic, antimony, bismuth, the additional compound being
such that the degree of oxidation of the doping element in this
additional compound is greater than or equal to 5; (b) Sintering of said
powder, the additional compound being such that the degree of oxidation
of the doping element can reduce during sintering.

Description:

TECHNICAL FIELD

[0001] This invention relates to an electrode for an electrochemical cell,
an electrochemical cell comprising such an electrode and a method of
making such an electrode.

STATE OF PRIOR ART

[0002] An electrochemical cell used particularly for electrolysers or fuel
cells at medium and high temperatures usually comprises two electrodes
between which there is a solid electrolyte.

[0003] A solid electrolyte is usually formed by a doped ceramic oxide that
at the working temperature is in the form of a crystalline lattice with
oxide ion vacancies. The associated electrodes are usually made from
cermets that comprise ceramic and metal. More precisely, the cermets used
in electrodes are composed for example of a perovskite mixed with a
metal. Perovskites are materials with an ABO3 or AA'BB'O6 type
crystalline structure in which A and A' are lanthanides or actinides and
B and B' are transition metals, based on the natural perovskite
CaTiO3 structure.

PRESENTATION OF THE INVENTION

[0004] The invention aims at disclosing an electrode with mixed electron
and proton conductivity, electron conductivity being better than with
electrodes according to prior art.

[0005] Another purpose of the invention is to disclose an electrode with
good adhesion to the solid electrolyte.

[0006] Another purpose of the invention is to disclose an electrode that
can be made at a lower temperature than electrodes according to prior
art.

[0007] To achieve this, a first aspect of the invention discloses an
electrode for an electrochemical cell with mixed electron and proton
conductivity, said electrode comprising a ceramic, said ceramic being a
perovskite doped with a lanthanide with one or several degrees of
oxidation, said ceramic being doped with an additional doping element
taken among the group composed of niobium, tantalum, vanadium,
phosphorus, arsenic, antimony, bismuth.

[0008] The fact that the ceramic is doped with niobium, tantalum,
vanadium, phosphorus, arsenic, antimony or bismuth makes the ceramic
capable of conducting electrons. The ceramic then conducts electrons and
protons, while if these doping elements are not present, the perovskite
doped with a lanthanide with a single degree of oxidation does not
conduct electrons.

[0009] Therefore, the invention can be used to make an electrode from a
material with the same nature as the solid electrolyte that has good
conductivity of both protons and electrons, even when the ceramic is not
mixed with a metal.

[0010] The electrode according to the invention can also have one or
several of the following characteristics taken individually or in any
technically possible combination.

[0011] The lanthanide is preferably chosen from among lanthanides with one
or several degrees of oxidation: ytterbium, thulium, dysprosium, terbium,
europium, samarium, neodymium, praseodymium, cerium, promethium,
gadolinium and holmium.

[0012] According to one embodiment, the electrode also comprises a metal;
the metal and the ceramic then form a cermet. The presence of this metal
can further increase the electronic conductivity of the electrode.

[0013] Advantageously, the perovskite used is a zirconate.

[0014] The lanthanide used is preferably erbium due to its size and
monovalence 3.

[0015] A second aspect of the invention also relates to an electrochemical
cell comprising two electrodes according to a first aspect of the
invention, and a solid electrolyte placed between the two electrodes.

[0016] Advantageously, the perovskite used in the solid electrolyte is of
the same nature as that used in the electrodes, which can give better
cohesion between the electrodes and the electrolyte. However, the
perovskite in the electrolyte will be doped with a lanthanide element
with a single degree of oxidation, while the lanthanide in the electrodes
may have one or several degrees of oxidation.

[0017] The electrochemical cell is advantageously an electrochemical cell
of an electrolysis device such as high temperature electrolysers
comprising a membrane with ionic conductivity. The invention is also
applicable to fuel cells, typically of the SOFC or PCEC type to which
technological developments of high temperature electrolysers are directly
applicable.

[0018] A third aspect of the invention relates to a method of making an
electrode based on the first aspect of the invention, the method
comprising the following steps:

[0019] (a) Synthesis of a perovskite
powder doped with a lanthanide with one or several degrees of oxidation;

[0020] (b) Synthesis of a powder of an additional compound comprising a
doping element taken among the group composed of niobium, tantalum,
vanadium, phosphorus, arsenic, antimony and bismuth, the additional
compound being such that the degree of oxidation of the doping element in
this additional compound is greater than or equal to 5;

[0022] (e)
Sinter this mix, the additional compound being such that the degree of
oxidation of the doping element can reduce during sintering.

[0023] Advantageously, the lanthanide that dopes the perovskite has a
single degree of oxidation when the electrolyte is manufactured, and one
or several degrees of oxidation when the electrodes are manufactured.

[0024] This method is particularly advantageous because the additional
compound provides oxygen to the mix of powders during sintering due to
the reduction in the degree of oxidation of the doping element during
sintering, so that sintering can be done in atmospheres that are not or
are only slightly oxidising (i.e. an almost non-oxidising atmosphere) and
at lower temperatures than is possible in methods according to prior art.

[0025] A non-oxidising or slightly oxidising atmosphere means an
atmosphere with a dew point of less than -56° C. and preferably
-70° C. A dew point of -70° C. corresponds approximately to
a pressure PH2O in H2O of 2.6×10-6 atm and a
pressure PO2 in O2 of 2.3×10-20 atm corresponding to
equilibrium at a sintering temperature of 1540° C.

[0026] Advantageously, the perovskite powder and the powder of the
additional compound are mixed with a metallic powder or a metallic phase
precursor so as to make a cermet, which can give an electrode with very
good electron conductivity.

[0027] If the electrode has a metallic phase, sintering is done under a
non- oxidising atmosphere.

[0028] Therefore, the method is capable of sintering under a non-oxidising
atmosphere at temperatures less than temperatures used in methods
according to prior art. For example, the sintering temperature of a
strontium zirconate doped with erbium under hydrogenated argon can be
reduced by 100° C. by the addition of 0.4 wt % of
ZnNb2O6.

[0029] Advantageously, the method also comprises a step (d) for compaction
of the mix between the mixing step (c) and the sintering step (e).

[0030] The invention also relates to a method of making an electrochemical
cell. In this case, the method according to the third aspect of the
invention also comprises a step between steps (c) and (e), and preferably
between steps (c) and (d), in which a stack is made comprising at least
two layers formed from a mix of the doped perovskite powder and the
additional compound, between which there is an interlayer comprising a
layer of perovskite powder.

[0031] The stack may also comprise two intermediate layers, each
intermediate layer being located between the interlayer and one of the
two layers formed from the mix of the doped perovskite powder and the
additional compound. These intermediate layers will be used either as a
protective layer of the electrolyte to prevent the diffusion of species
between the electrodes and the electrolyte, or as accommodation layers if
there are differences between the coefficients of thermal expansion of
the electrode and electrolyte layers, particularly due to the presence of
metal in the electrodes.

[0032] A fourth aspect of the invention relates to a method of
manufacturing an electrode based on the first aspect of the invention,
the method comprising the following steps:

[0033] (a) Direct synthesis
of a perovskite powder doped with a lanthanide with one or several
degrees of oxidation containing an additional compound comprising a
doping element taken among the group composed of niobium, tantalum,
vanadium, phosphorus, arsenic, antimony, bismuth, the additional compound
being such that the degree of oxidation of the doping element in this
additional compound is greater than or equal to 5;

[0034] (b) Sintering
of said powder, the additional compound being such that the degree of
oxidation of the doping element can reduce during sintering.

DESCRIPTION OF THE FIGURES

[0035] Other characteristics and advantages of the invention will become
clearer after reading the following detailed description given with
reference to the appended figures that show:

[0036] FIG. 1, a diagrammatic representation of an electrochemical cell
according to one embodiment of the invention;

[0037] FIG. 2, a diagrammatic representation of the steps in a method
according to the invention.

[0038] Identical or similar elements are marked by identical reference
symbols in all figures, to improve clarity.

DETAILED DESCRIPTION OF AT LEAST ONE EMBODIMENT

[0039] FIG. 1 shows an electrochemical cell according to one embodiment of
the invention. This electrochemical cell comprises two electrodes 1, 3
between which there is a solid electrolyte 2. Each electrode 1, 3 is an
electrode according to the first aspect of the invention.

[0040] Each electrode 1, 3 is made from a ceramic material that is a
perovskite doped with a lanthanide. In this example, the perovskite is a
zirconate with formula AZrO3. The zirconate is dope by a lanthanide
that in this case is erbium. Furthermore, the perovskite doped with the
lanthanide is doped with a doping element from among the group composed
of niobium, tantalum, vanadium, phosphorus, arsenic, antimony and
bismuth. These doping elements are chosen to dope the ceramic because
they can change from a degree of oxidation equal to 5 to a degree of
oxidation of 3, which releases oxygen during sintering as we will see
later. More precisely, the doping element is preferably niobium or
tantalum. Each electrode may also comprise a metal mixed with ceramic to
form a cermet.

[0041] In this example embodiment, the ceramic comprises between 0.1% and
0.5% by mass of niobium, between 4 and 4.5% by mass of erbium and the
remainder in zirconate.

[0042] The electrochemical cell in FIG. 1 is manufactured according to the
method described with reference to FIG. 2. The first step is to
synthesise a perovskite powder doped with a lanthanide during a step 101.
The ceramic thus obtained is in the form of large aggregates composed of
nanometric grains. This ceramic is then formulated to reduce the size of
its grains to obtain a grain size distribution that will be conducive to
compaction of the powder.

[0043] A powder of an additional compound comprising a doping element from
among the group composed of niobium, tantalum, vanadium, phosphorus,
arsenic, antimony and bismuth, is also synthesised during a step 102, the
additional compound being such that the degree of oxidation of the doping
element is greater than or equal to 5 in this additional compound. This
additional compound may for example by a niobiate, in other words a
compound comprising niobium, or a tantalate, in other words a compound
comprising tantalum. The niobiate used may for example be zinc niobiate
with formula ZnNb2O6.

[0044] The next step is to mix the doped perovskite powder obtained in
step 101 and the powder of the additional compound obtained in step 102,
in a step 103. This mix may for example comprise between 0.1% and 0.5% by
mass of zinc niobiate.

[0045] The mix thus obtained can then be mixed with a metal powder so as
to form a cermet, during a step 104.

[0046] A step 105 can then be made to form a stack that will subsequently
form the electrochemical cell and that comprises two layers formed from
the mix of doped perovskite powder and the powder of the additional
compound, between which there is an interlayer comprising a layer of
perovskite powder. The two layers formed from the mix of doped perovskite
powder and the powder of the additional compound will each form the
electrodes of the electrochemical cell, while the interlayer will form
the solid electrolyte. The stack may also comprise two intermediate
layers, each intermediate layer being placed between the interlayer and
one of the two layers formed from the mix of the doped perovskite powder
and the additional compound. These intermediate layers will act either as
the electrolyte protective layer to prevent diffusion of species between
the electrodes and the electrolyte, or as accommodation layers if there
are any differences between the coefficients of thermal expansion of the
electrode and electrolyte layers, particularly due to the presence of
metal in the electrodes.

[0047] The stack thus obtained can then be compacted during a step 106,
and then sintered during a step 107.

[0048] The manufacturing process is particularly advantageous because the
degree of oxidation of the doping element will reduce during sintering,
usually from +5 to +3, such that the additional compound releases oxygen.

[0049] It is thus possible to sinter at a lower temperature due to this
added oxygen. Thus for example, if the perovskite used is a zirconate
doped with erbium and mixed with zinc niobiate, sintering can take place
at 1415° C.

[0050] Advantageously, sintering is done under a reducing atmosphere, in
other words an atmosphere of hydrogen (H2) and argon (Ar).

[0051] The electrode thus obtained has good cohesion with the electrolyte.

[0052] The electrode thus obtained also has enhanced electron conductivity
and good proton conductivity. The ratio of electron conductivity to
proton conductivity of the electrode thus obtained is equal to
approximately 100.

[0053] Naturally, the invention is not limited to the embodiments
described with reference to the figures, and variants could be envisaged
without going outside the scope of the invention. In particular, the
proportions of the different materials are given only for illustration.
The geometry of the electrochemical cell could also be different from the
disclosed geometry.

Patent applications by Baroudi Bendjeriou, Saint-Etienne FR

Patent applications by Beatrice Sala, Saint Gely Du Fesc FR

Patent applications by Dominique Goeuriot, Monistrol Sur Loire FR

Patent applications by Frédéric Grasset, Montpellier FR

Patent applications by Hisasi Takenouti, Ollainville FR

Patent applications by Centre National De La Recherche Scientifique (CNRS)